WHAT PLANTS NEED 301 



8. Light. When it is said that light is necessary for 

 plant work, this does not mean that bright sunlight is neces- 

 sary. Very ordinary light seems to be sufficient, and certain 

 experiments indicate that very much less light than plants 

 usually receive is not injurious. It is evident, therefore, 

 that cultivated plants are not likely to lack light. It is not 

 so much a question, however, of the total amount of light 

 as of the presence of certain active rays of light that plants 

 use in the manufacture of food. When light passes through 

 smoke, its usefulness to plants is much diminished, and this 

 becomes a serious problem in the neighborhood of factories, 

 and in smoke-ridden towns and cities. When light has 

 passed through foliage, the necessary active rays have dis- 

 appeared, so that plants cannot grow completely shaded by 

 other plants. This is recognized in a general way when the 

 scarcity of low vegetation in a dense forest is noted ; but even 

 the densest forest lets some unscreened light through, and does 

 not form so complete a shade as many a cultivated crop forms. 

 It is a wise thing, therefore, to see to it that plants under 

 cultivation are not densely shaded by other plants. 



9. Heat. The favorable condition of temperature varies 

 for different plants and for the different periods of the same 

 plant. In general, a lower temperature is more favorable for 

 a seedling than for a maturing plant, and this fact is recog- 

 nized in the spring sowing and summer harvesting of ordinary 

 crops, as wheat. The variation in the temperature require- 

 ments of different seedlings also explains why some seeds 

 are planted earlier in the season than others. The successful 

 extension of crops into different latitudes has depended upon 

 securing varieties with different temperature ranges. The 

 results of passing beyond the best temperature range in either 

 direction are soon obvious in a plant. If the temperature is 

 too low, the plant grows very slowly; if it is too high, the 

 plant becomes unusually tall and slender ; and in both cases 

 its lack of vigor is shown by the fact that it is unusually 



